Whilst screening machines, especially vibratory screening machines such as the so called ‘shale shakers’ of the oil well drilling industry are used with success in methods of solids/liquids separation, especially classification, there is a need to improve throughput and effectiveness.
During the drilling of an oil well, fluid known as mud is circulated, under pressure, inside the drilling assembly to the drill bit. One of the functions of the drilling mud is to carry the rock cuttings generated during the drilling process at the drill bit, out of the borehole.
When the drilling mud arrives at the drilling rig after use in drilling, the solids fraction of the mud will contain desirable solids and drilled solids. The drilled solids are generally undesirable solids comprised predominantly of rock but can contain metal fragments. The drilled solids are undesirable as these are generally rock cuttings that if allowed to accumulate at increased concentrations result in undesirable effects on the fluid properties of the mud. As the concentrations of drilled solids in a mud increases the fluid properties are affected until the mud becomes unusable and requires replacement or the addition of new mud to dilute the concentration of drilled solids such that the desired fluid properties are restored. The removal and control of the concentrations of drilled solids is generally regarded as a most important activity in contributing to the successful, safe and economic drilling of an oil well, within the planned time and cost. The process of recycling used drilling mud should remove drilled solids (at least above a selected size range) while leaving desirable solids such as weighting material within the fluid.
Drilled solids are conventionally removed from the mud by using first shale shakers to screen the fluid. Rock cuttings above screen size are removed during screening and the fluid passes into storage tanks for subsequent mechanical and chemical processing, where this is desirable, and ultimate recirculation to the oil well. After screening at the shale shaker, additional solids separation techniques can be applied to remove any drilled solids that have passed through the shale shaker, being smaller than the screen size fitted to the shale shaker.
The drilling mud returning to the drilling rig from a well normally contains a low concentration of drilled solids within a large volume of fluid. The drilled solids removal system is thus required to process a large volume of fluid to remove a small volume of drilled solids. Consequently the size of a drilled solids removal system has historically been directly relative to the volume of fluid to be processed and not the volume of solids to be removed.
One or more shale shakers are used depending upon the volume of fluid being pumped and the separation efficiency required. Generally as finer screens are fitted to the shale shaker the process capacity of the shaker decreases while the efficiency of separation of solids increases. Typically screening will take place using screens (screening material), generally made of woven wire mesh, of between 10 and 400 mesh. These screens will contain between 10 and 400 wires per inch respectively and aperture hole size will vary according to the weave pattern and diameter of the wire used in the weave.
To achieve the required process capacity and separation efficiency, a drilling rig shale shaker installation will typically contain between one and eight shale shakers, although some installations can employ more machines.
The need to design a vibratory screening machine to provide the required fluid throughput while transporting solids to the point of discharge from the screen has resulted in conventional machines being of a larger size or used in greater numbers than is ideal where space and weight are restricted by either physical or economic factors.
Despite the advent of improved screening machines such as high capacity, multi deck shale shakers to improve the throughput, and ability to recycle solids of selected sizes, there is still a need for yet further improved equipment and methods to allow increased separation efficiency and/or modes of operation.
In WO2102/140398 screen assemblies for use in vibratory screening machines such as shale shakers are described. These screening assemblies include first and second screen units one above the other in use and spaced apart by a support frame disposed in between. The screen units include screen panels of screening material through which a feed such as a liquid and solids mixture can be processed. These screening assemblies can increase effectiveness of a given screening machine as a single screening assembly provides two screening surfaces, one above the other through which a liquid and solids mixture can be successively processed. Despite these improvements there remains a need for improved equipment and methods to allow increased separation efficiencies and/or further modes of operation.